The present invention relates to clock generators, and more specifically, it relates to a clock generator for deriving clock information from a digital bit stream. The invention is particularly suitable for demodulating an eight-to-fourteen modulation (EFM) bit stream which is derived from digital audio discs such as "compact discs".
In compact disc recording a digital modulation technique, known as eight-to-fourteen modulation (EFM), is employed whereby original eight bit codes of random clock spacings are converted to fourteen bit codes of the format having a minimum spacing of three clock pulses and a maximum spacing of eleven clock pulses to provide enough tracking information for laser beam to follow tracks and enough clock information for disc players to demodulate the EFM bit stream. The fourteen-bit codes are formatted into a series of data blocks, or "frames" each being identified by a 24-bit frame sync code composed of consecutive series of eleven bits of "1"s and eleven bits of "0"s plus two binary "1"s.
Conventional clock generators employed in compact disc players include circuitry that detects a series of data bits having the minimum and maximum clock spacings and counts clock pulses generated by a voltage-controlled oscillator which are present during the minimum and maximum spacings of the detected data bits. Two count values are derived as a measure of the frequency of the clock pulse and used to control the operating frequency of the oscillator. One disadvantage of the prior art is that it requires many circuit components with a resultant increase in cost. Another disadvantage resides in the fact that since the data bits of minimum and maximum clock spacings occur at random the phase-locked loop is likely to remain out-of-phase with the input bit stream for a substantial period if successive frames contain no data bits having mininum and maximum clock spacings.